In order to control the attitude of satellites, it is general practice to use a system of reaction wheels or of inertia wheels.
Such a system comprises three main portions, namely: a stator secured to a structure which is itself fixed within the satellite; a rotor spinning at high speed about an axis; and means for driving the spinning portion, i.e. the rotor.
In older systems, the rotor rotates about a solid shaft, i.e. a rotary shaft with ball bearings or a similar member.
In more recent systems, use has been made of a technique that is more advantageous, implementing suspension by means of a magnetic and/or electromagnetic bearing under servo-control.
The rotary portion, i.e. the rotor, is spaced apart from the stator by clearance that is typically a fraction of a millimeter in normal operation.
The main advantage conferred by this technique lies in the fact that the rotor “floats” and thus rotates under magnetic levitation without coming into contact with the stator.
A priori there is no friction, or at worst friction is very low. As a result losses due to friction are also extremely low and wear is virtually non-existent.
Furthermore, and unlike systems using shafts and ball bearings, there are no harmful effects resulting from the need to lubricate such members: lubrication deteriorates over time, in particular on long-duration missions, it is sensitive to very low temperatures if they occur for very long periods, and/or to temperature variations that are fast and very large.
It may also be mentioned that magnetically suspended systems present levels of microvibration and of noise emission that are very low, and this is most favorable, in particular for scientific missions or for earth observations missions.
Until recently and in spite of the advantages they provide, such systems, which are of relatively large size, have been used only for missions of long duration, and they have been installed only on satellites that are likewise of large dimensions. Progress specifically in miniaturization and in large-scale integration of the electronic circuits for the above-mentioned servo-control has made it possible to greatly reduce the size of such systems, thereby enabling them to be integrated in satellites of small size, and in particular for short missions.
By way of non-exhaustive example, a magnetically suspended wheel is described in the article by Michael Scharfe et al. entitled “The challenges of miniaturization for magnetic bearing wheel”, published in “Proc. 9th European Space Mechanisms and Tribology Symposium”, ESA-SP-480, Sep. 19-21, 2001, pp. 17-24. That article refers in particular to work carried out at the Dresden Institute of Precision Engineering. Reference can advantageously be made to that article for a more detailed description, and in particular by referring to its FIG. 3 which is a diagram showing an example of a magnetic suspension.
In spite of the advantages recalled above, magnetic suspension systems nevertheless present at least one drawback.
The rotor is not rigidly connected to the stator, but on the contrary it is free to move both in parallel with and orthogonally relative to its axis of rotation, even if the amplitude of such movement is limited. As a result it is only (electro) magnetic forces that act on the rotor to keep it in a suitable three-dimensional position and to prevent it from coming into contact with the stator. In the absence of such forces, there is a risk of damage to the various members, both static and rotary.
Thus, it is general practice to provide an emergency mechanical bearing so as to enable the rotor to bear against the stator without the magnetic suspension members coming into mechanical contact while the rotor is not magnetically suspended.
Nevertheless, while the system is being subjected to high levels of stress, e.g. during launching of a satellite (intense vibration, acceleration, etc.), the above-mentioned emergency mechanical bearing is insufficient for withstanding the effects of those stresses without damage, it naturally also being understood that the magnetic suspension cannot be put into operation while the satellite is being launched.
It is therefore necessary to provide additional members that serve to lock the rotor completely relative to the stator. Under other circumstances, it is also necessary to lock the rotor, in particular while the system is being transported or handled.
Such locking must naturally be only temporary. In particular, after launch or when it is desired to perform tests on the system to ensure that is operating properly, it is necessary to unlock the rotor and to begin a stage of “normal” operation (under magnetic suspension).
Various solutions are provided for this purpose in the prior art. Most such solutions rely on the presence of a “consumable” component in the system, i.e. a component for single use only. It is the component which serves to lock the rotor temporarily relative to the stator. It will readily be understood that that solution presents a major drawback. The component needs to be replaced each time it has been used.
After it has performed its function, i.e. once the satellite has been launched and put into orbit, it is of no further use. Under such circumstances, the fact that the component is for single use only is not, a priori, a major drawback. Nevertheless, it should be observed that certain precautions need to be taken, since the above-mentioned component must not interfere with normal operation of the system (while it is under magnetic suspension).
In addition, and above all, the system is normally subjected to a certain amount of testing on the ground, and to intermediate handling operations. After each such test, it is therefore necessary to replace the single-use component. This generally also requires special maintenance operations to be performed that go beyond mere application of electrical or electronic controls.
Some of the main proposed solutions are summarized briefly below.
U.S. Pat. No. 4,345,485 (Jean-Luc Livet et al.) describes a locking mechanism in which the rotor is retained by temporarily eliminating the axial clearance between the rotor and an emergency mechanical bearing of the inertia wheel. The system has two emergency bearings each presenting a conical bearing surface capable of coming into mechanical contact with a complementary conical bearing surface made on the rotor. One of the complementary bearing surfaces can be caused to move axially. For this purpose, this bearing surface is associated manually with an axially sliding guide. In orbit, in order to release the rotor, the guide is sheared using shears controlled by a pyrotechnic charge. A spring acts on the rotor in order to retract it. The mechanism is thus a temporary locking device that acts axially.
U.S. Pat. No. 4,566,740 (Jean F. Beau et al.) describes a locking mechanism in which an axial cable is used for directly eliminating the axial clearance of a flywheel. The rotor is pressed against the surface of an emergency bearing where it is blocked. The rotor is released by using a pyrotechnic charge to cut the cable. That mechanism is likewise a temporary locking device that acts axially.
U.S. Pat. No. 4,872,375 (Hubert Vaillant de Guelis et al.) describes a locking mechanism using a cable that forms an annular loop in a plane extending transversely to the axis of rotation of the rotor. The cable acts radially on a plurality of individual radially-movable bearing surfaces associated with the stator. The rotor has a second radial bearing surface, and under the action of radial forces it engages in the bearing surfaces of the stator and becomes blocked. The rotor is released by means of a cable cutter. That constitutes a temporary locking device that acts radially.
As will readily be understood, all of those mechanisms are for single use only and therefore present the drawbacks outlined above.
A multiple-use mechanism is also known.
The article by U. Bichler and T. Eckardt entitled “A gimbaled low noise momentum wheel” published in “27th Aerospace Mechanisms Symposium”, “NASA Conference Publication 3205”, May 12-14, 1993, page 196 describes an inertia wheel having magnetic suspension and including a radially-acting pneumatic mechanism for locking the rotor. It comprises two rubber tubes as can be seen more particularly in its FIG. 4. The mechanism serves to lock the rotor by inflating the tubes with gas under pressure. In orbit, the rotor is released by means of valves that are actuated by solenoids. The multiple-use mechanism retains the rotor in a safe position during launch and during stages prior to testing.
A priori, it might be thought that that mechanism solves the problem of multiple use. It does not require a single-use component.
Nevertheless, experience shows that it is not without its own drawbacks. Specifically, firstly the tubes need to be refilled after each occasion the rotor is unlocked, and secondly, after a long period of storage, it is often found that it is also necessary to refill the tubes since they have become deflated due to residual leaks.